Long range and local ordering patterns in the disordered layered calcium aluminosilicate hydrate strätlingite

Strätlingite (Ca 2 Al 2 SiO 2 (OH) 10 ·2.25H 2 O), a mineral rarely found in nature, which is believed to be responsible for their long-term stability due to its lamellar microstructure that hinders crack propagation along the interfaces between the constituents of concrete. However, the identification of the mineral during early hydration times is often complicated by low crystallinity and absence of a reliable disorder-free structural model. Published crystal structures of strätlingite often contain positionally-disordered atoms in both brucite-like layer and half-occupied double-tetrahedra (DT) layers. In this work we reinvestigate the average structure of strätlingite combining bulk high resolution powder X-ray diffraction (XRPD) and 3D electron diffraction (3D-ED), demonstrating that the R 3 ¯ space group describes the brucite-like layer of strätlingite more appropriately than R 3 ¯ m , without positional disorder of the oxygen atoms. Partial occupation in the DT-layer is addressed via solid state nuclear magnetic resonance (ssNMR) and pair distribution function (PDF) analysis, showing that the condensation of Si(Al)O 4 tetrahedra is best described by a Q 3 configurational arrangement in the form of deformed double six-membered rings, exhibiting local displacements within the ab -plane.